First, a grindstone prepared by bonding superabrasive grains of diamond or CBN with a metal, resin or a vitrified bond is known as a superabrasive grindstone which is a kind of superabrasive tool. Further, a grindstone prepared by holding and fixing superabrasive grains on a base by electroplating is known as a superabrasive grindstone in the form of holding superabrasive grains in a single layer. Such a superabrasive grindstone is called an electroplated superabrasive grindstone. The grains are generally fixed onto the base to such a degree that the superabrasive grains come into contact with each other, and hence the degree of concentration of grains may be too high, depending on the purpose of grinding performed with this grindstone. As a countermeasure therefor, means are employed for improving the flow of a grinding fluid and eliminating chips, such as a method of locally inhibiting electroplating by a method of (1) providing grinding grooves on the grinding surface of the grindstone or (2) locally applying an insulating paint to the base, and locally forming a part having no superabrasive grains on the grinding surface.
On the other hand, the thickness of a plating layer is rendered at least 1/2 the diameter of the superabrasive grains, in order to ensure holding power for the superabrasive grains.
With respect to the aforementioned electroplated superabrasive grindstone, a superabrasive grindstone in which superabrasive grains are fixed onto a base by a brazing filler metal layer is known. As to diamond abrasive grains, for example, the so-called brazing method utilizing such a characteristic that an alloy consisting of nickel, cobalt and chromium or an alloy consisting of silver, copper and titanium readily wets surfaces of diamond abrasive grains and directly fixing diamond abrasive grains onto a base by employing this alloy is also known.
Further, a porous resin bond grindstone employing fine diamond grains is proposed as a grindstone for attaining working of high accuracy and a high grade. Increase of chip pockets or the like is aimed to be achieved by a porous part in this grindstone.
Surface roughness of a ground surface is regarded as being decided by the effective abrasive grain number per unit surface area of the grindstone. However, how to grasp the effective abrasive grain number with respect to the grain sizes and the degree of concentration of the abrasive grains is not necessarily clear, and there has been the following problem depending on the levels of the grain sizes of the abrasive grains.
In a grindstone employing abrasive grains having relatively large grain sizes, i.e., coarse grains, holding power for the abrasive grains is strong, fewer abrasive grains are dropped out of the grindstone and the flow of a grinding fluid is also excellent. However, the accuracy of a surface ground by coarse grains is low and its surface roughness is large. In a grindstone employing abrasive grains having relatively small grain sizes, i.e., fine grains, on the other hand, it is possible to increase the accuracy of a ground surface and to reduce its surface roughness. However, holding power for small abrasive grains is weak, more abrasive grains are dropped, and the flow of the grinding fluid is also inferior. In the grindstone employing fine grains, therefore, grinding performance is low, the abrasive grains become ungrindable following slight wear, and the life of the grindstone is short.
To prepare a diamond rotary dresser, i.e. a kind of superabrasive tool, it is well known to fix diamond abrasive grains to the outer peripheral surface of a cylindrical base in a single layer, as disclosed in Japanese Patent Laying-Open No. 59-47162, for example.
Another example of a known diamond rotary dresser is disclosed in Japanese Patent Publication No. 1-22115. These diamond rotary dressers, having wide acting ranges, are employed for dressing a conventional grindstone of WA or GC (type of JIS) or a CBN grindstone with high accuracy. Means for densely fixing diamond grains onto a base, flattening surfaces acting on dressing by truing forward end portions of the diamond grains and improving dressing accuracy are various means employed by the diamond rotary dresser.
However, the formation of flat surfaces on the forward end portions of the diamond grains lowers the sharpness of the diamond rotary dresser. Thus, the dressing resistance increases when a conventional grindstone of WA or GC or a CBN grindstone is dressed. Consequently, there has been such a problem that vibration takes place in dressing and the vibration exerts a bad influence on shaping accuracy of the grindstone, i.e., transfer accuracy to the grindstone.
Further, a superabrasive lap surface plate is a kind of superabrasive tool. Recently, improvements in the accuracy of flatness and parallelism of a workpiece is required in lapping, due to rapid technological innovation such as high integration in a semiconductor device or superprecision in metal working or ceramics working. This results in demands of greater accuracy not only of the lapping machine employed for this working, but also intensifies the requirement of accuracy and characteristics for the lap surface plate.
Lapping refers to a method of working a surface by supplying free abrasive grains mixed into a lap liquid between a lap surface plate and a workpiece, rubbing the lap surface plate and the workpiece with each other while applying pressure, scraping the workpiece by rolling action and scratch action of the free abrasive grains and obtaining a high accuracy surface.
The lap surface plate employed for conventional lapping is made of cast iron. For example, a lap surface plate of spherical graphite cast iron is generally employed for lapping on a silicon wafer. The lap surface plate must have such properties that ensure that it is capable of maintaining accuracy of a flat surface over a long period, that the material is homogeneous without irregularity in hardness, without casting defects that will cause scratching on the surface of the workpiece, and with a holding ability for abrasive grains. In order to satisfy the above necessary conditions, cast iron is generally employed as the material for the lap surface plate.
In conventional lapping, however, a great many free abrasive grains are consumed, and hence, great volumes of mixtures of used free abrasive grains, chips and a lap liquid, i.e., sludge are generated. As a result deterioration of working environment and occurrence of environmental pollution have become a significant subject of discussion.